Causes / typical injury mechanism: Patellar sleeve fracture occurs within the pediatric population. The patella is a sesamoid bone that is part of the quadriceps extensor mechanism; ossification begins at 3-6 years of age. This type of fracture is described as an avulsion of a small bony fragment, the articular cartilage, periosteum, and retinaculum, from the distal pole of the patella. During growth, the lower pole of the patella is inherently weak at the growth plate, which can lead to avulsion of the lower pole of the patella.

This type of osteochondral fracture of the patella is the result of high-impact jumping activities where the quadriceps contracts on a flexed knee (eccentric contraction). This leads the inferior pole of the patella to be avulsed from the rest of the patella's body. The sleeve fragment will be pulled inferiorly by the patellar tendon, and the patellar body will be pulled superiorly by the quadriceps tendon.

Classic history and presentation:

• A common feature in the patient's history of patellar sleeve fracture is the absence of direct knee trauma.
• The patient will describe their activity leading to injury as associated with sports or jumping activities.
• After the injury, the patient's knee joint will be swollen and painful, and the patient will be unable to perform a straight leg raise.

Prevalence: The incidence of patellar fracture among the pediatric population is at 6.5%; patellar sleeve fracture is the most common (38%-73%) type of patellar fracture among skeletally immature children. Patellar sleeve fractures are rare among the pediatric population (less than 1% of all pediatric fractures). This is because the immature patella is surrounded by a thick layer of cartilage, which protects it from fracture. Patellar sleeve fractures are also rare because children are less likely to undergo high-energy trauma.

• Age – 8-12 years
• Sex / gender – More common in males (5:1)

Risk factors: Patellar sleeve fracture risk factors are sports with jumping (eg, basketball, gymnastics, hurdling), landing on feet from a slight elevation (eg, monkey bars), and jumping activities. Patellar sleeve fracture is unique to the pediatric population between the ages of 8 and 12 years, where the patella still has open growth plates, making it susceptible to avulsion forces prior to skeletal maturity.

Pathophysiology: Patellar sleeve fracture is the separation of the articular cartridge, retinaculum, periosteum, and small bony fragments of the patella. This is due to vigorous contraction of the quadriceps to a flexed knee, which leads to avulsion forces. This leads to the separation of the cartilage "sleeve" from the patella and thereby disrupts the extensor mechanism.

Grade / classification system:

• Inferior patellar pole – High-riding patella (patella alta) is the most common patellar sleeve fracture
• Superior patellar pole – Low-riding patella (patella baja) with an anterior tilt